The response to accelerated heavy ions of a rhabdomyosarcoma tumor system in an inbred rat strain is being studied using a number of different endpoints. Radiation-induced growth delay of the tumor, proliferating capacity of tumor cells irradiated either in vivo or in vitro, both in the oxygenated and hypoxic state, and cell kinetics after irradiation are being studied. These projects will be continued with various heavy ion beams having ranges and Bragg-peak widths of interest to the radiotherapist. Fractionation studies will be expanded to include measurements of repopulation of the surviving cells as the fractionation proceeds. Mixed modalities of low and high LET radiation will be studied to evaluate the efficacy of using such schedules to improve tumor control. Studies will be initiated on the response of this tumor system in vivo to the combined action of high LET beams and hypoxic cell sensitizers. Tumor model development will continue to include the information gained from the experimental program.